Light sources have three main attributes. The optimization of each determines whether stencil quality is excellent, good, or poor.

Spectral output The wavelengths of light, or spectral output, of an exposure source should match closely the spectral sensitivity of photostencil materials. The closer the match, the more efficient the light source, and the better it will cure or crosslink the molecules of the stencil material so that the stencil will be more durable. Common sensitizers used in photo films and emulsions have specific sensitivity ranges. It’s a common misconception that photographic stencil materials are UV-sensitive. In fact, they are sensitive to light ranging from upper ultraviolet through the visible blue part of the spectrum. That’s why yellow lights make good safelights—photostencil materials don’t react to the longer wavelengths of yellow.

Light intensity Photostencil materials, even those comparatively fast ones formulated for projection exposure, are not nearly as fast exposing or sensitive as camera film. If photographic stencil materials were made that fast, they would be virtually impossible to protect from pre-exposure. Thus, light sources need the power—the intensity—to expose stencils in a reasonably productive amount of time.

Light geometry Light geometry concerns the angle of incidence of light rays as they strike the artwork and pass through its clear portions into the photostencil material. Light rays emitted from a small area or point source—at a minimum distance of 1.5 times the diagonal of the stencil—are more nearly perpendicular as they meet the artwork and stencil (perpendicular light incidence). This affords better transfer of image and non-image information from the artwork into the photostencil material.

By contrast, with a widely dispersed light source such as fluorescent tubes, light rays enter the stencil material at oblique angles, quite literally angled behind the dark edges of the artwork, reducing the fine detail or resolution of the stencil (wide angle incidence).

Light sources are categorized according to how well they provide these three key exposure attributes. The highest quality light sources are metal halide or carbon arc. Mercury vapor and pulsed xenon are of medium quality. Quartz lamps, fluorescent tubes, flood bulbs, etc., have problematic deficiencies, though they can be quite adequate for low-resolution work—or in shops that don’t need high stencil throughput.
All is not lost if you have a poor light source, but you will have to work smart to compensate for it and understand and accept the limitations imposed by it.